Computer systems that capture, editing and playback motion video typically process motion video data as digital data, representing a sequence of digital images. Such data typically is stored in computer data files on a random access, computer readable medium. An image may represent a single frame, i.e., two fields, or a single field of motion video data. Such systems generally allow any particular image in the sequence of still images to be randomly accessed for editing and for playback.
Since digital data representing motion video may consume large amounts of computer memory, particularly for full motion broadcast quality video (e.g., sixty field per second for NTSC and fifty fields per second for PAL), the digital data typically is compressed to reduce storage requirements. There are several kinds of compression for motion video information. One kind of compression is called “intraframe” compression, which involves compressing the data representing each image independently of other images. Commonly-used intraframe compression techniques employ a transformation to the frequency domain from the spatial domain, for example, by using discrete cosine transforms, to generate a set of coefficients in the frequency domain that represent the image or portions of the image. These coefficients generally are quantized, placed in a specified order (commonly called a zig-zag ordering), then entropy encoded. Entropy encoding is a lossless process that typically involves generating code words that represent the coefficients, using a form of Huffman coding scheme. Image quality of compressed images is primarily affected by the loss of information through quantizing.
Some compression techniques involve additional operations that further affect image quality. For example, some compression techniques reduce the size of an image before it is transformed and quantized. Some other compression techniques reduce the bit depth, by rounding, for example, from 10-bits to 8-bits.
More compression can obtained for motion video sequences by using what is commonly called “interframe” compression. Interframe compression involves predicting one image using another. This kind of compression often is used in combination with intraframe compression. For example, a first image may be compressed using intraframe compression, and typically is called a key frame. The subsequent images may be compressed by generating predictive information that, when combined with other image data, results in the desired image. Intraframe compressed images may occur every so often throughout the sequence. For interframe compressed image sequences, the interframe compressed images in the sequence can be accessed and decompressed only with reference to other images in the sequence.
Compression techniques for video also may provide a variable bit rate per image or a fixed bit rate per image. Either type of technique generally uses a desired bit rate in a control loop to adjust parameters of the compression algorithm, typically parameters for quantization, so that the desired bit rate is met. For fixed bit rate compression, the desired bit rate must be met by each compressed image or by the compressed data for each subset of each image. For variable bit rate compression, the desired bit rate is generally the average bit rate (in terms of bits per image) that is sought.